There are a variety of electrolytic processes that use gas-evolving electrode(s), such as copper electrowinning, zinc electrowinning, manganese electrowinning, electrogalvanizing, copper foil production, metal finishing, and metal recovery. In these processes, the rate of metal deposition is proportional to the current, but significant energy is expended owing to an over-potential at the gas evolving anode. The application of an electrical potential across the electrodes causes the movement of ions in the electrolyte and the movement of electrons from the anode to the cathode to complete the electrical circuit. This flow of electrons is accomplished by the removal of electrons from the anode with the supply of electrons at the anode provided by negatively charged anions, such as chloride, or reducing agents in the electrolyte solution.
Some anions, such as sulfate, are not discharged directly from aqueous solutions, so completion of the circuit in this instance requires the use of a reducing agent as the source of electrons. The potential required for the discharge of this reducing agent is dependent on the reversible (equilibrium) potential of that particular species. In a sulfuric acid/copper sulfate electrolyte for example, using an insoluble lead/lead oxide anode, water is the primary source of electrons for completion of the circuit with the subsequent evolution of oxygen gas.
In practice, the actual voltage required to produce gas evolution at the anode is considerably higher than just the equilibrium potential. This difference is called the overvoltage, and is caused by the irreversible reaction of the formation of oxygen bubbles on the surface of the anode. This overvoltage can increase the operating potential of the cell by as much as 1 volt, depending on the specific electrode array in use. Since energy consumption in electrolytic processes is directly proportional to the operating potential of the cell, lowering the cell voltage will have a significant impact on the energy consumption and corresponding manufacturing costs related to these electrolytic processes.
Reducing the energy requirement of the electrolytic cell is also desirable. There have been attempts to reduce the overall energy requirement of the electrolytic process. For example, the oxidation potential at the anode of an electrowinning cell has been decreased by using titanium anodes coated with a layer containing platinum metals (i.e., “dimensionally stable anodes” (DSA)) instead of lead/lead oxide anodes. But DSA anodes are relatively expensive.
Inorganic materials have previously been added to electrowinning processes to reduce the overall cell voltage in addition to reducing oxygen gas byproduct, thus reducing acid mist. For example, an Fe(II)/Fe(III) couple, combined with sulfur dioxide to reduce Fe(III) to Fe(II), was used to decrease cell voltage from 2.00 V to 0.94 V and also to reduce acid mist. But these effects were only seen using the DSA anodes, not with the more prevalent lead/lead oxide anodes. (See, S. P. Sandoval et al., “A Substituted Anode Reaction for Electrowinning Copper,” Proceedings of COPPER 95-COBRE 95 International Conference, Volume III—Electrorefining and Hydrometallurgy of Copper, edited by W. C. Cooper, et al., pp. 423-435).
Another problem inherent with the use of gas-evolving anodes is that gas is produced, which agitates the electrolyte solution and may cause acid mist. Acid mist may be harmful to the health of the electrolytic process workers. Another problem with some electrolytic processes is the amount of electrical potential or energy required, and the related expense.
An electrolytic process generally takes place in an electrolytic cell. An electrolytic cell typically comprises at least one anode and at least one cathode. The anode(s) and cathode(s) (or electrodes) are in contact with an electrolyte solution. Gas-evolving anodes are insoluble in the electrolyte solution. A highly pure metal is deposited on a cathode.
During these electrolytic processes, metal is reduced at the cathode(s) and oxidation occurs at the insoluble anode(s). Gas bubbles may be formed at the anode(s) and may rise upwardly toward the electrolyte solution surface and burst, thereby forming a mist aerosol of finely dispersed electrolyte droplets. This mist, or aerosol, then typically spreads throughout the area where the electrolytic cells are operated, sometimes called the tank house. The composition of the mist is dependent on the composition of the electrolyte solution, and typically contains sulfuric acid and metal salts. The acid mist is corrosive to equipment and may be a health hazard. The acid mist may cause extreme discomfort to the skin, eyes, and respiratory systems of tank house workers.
Thus, various methods have been used to either contain or inhibit the generation of acid mist by electrolytic cells. But no method is completely satisfactory.
One common method used to address the problem of acid mist is to use a powerful ventilation unit to remove contaminated air from a tank house. The air is then circulated through a scrubber to remove contaminants before it is either recirculated or released into the environment. This method consumes a lot of energy and is not very effective at removing the acid mist.
Another common attempt to suppress acid mist formation has been to use mechanical interference devices. Common examples of mechanical interference devices are layers of floating plastic balls, beads, discs, rods etc., in the electrolyte solution. These devices create a surface where gas bubbles can burst less violently and also provide a surface where the acid mist from the bursting gas bubbles may be collected and drained back into the electrolyte solution.
Another method of addressing the acid mist problem has been to apply certain fluorochemical surfactants to form a “foam blanket” on the surface of the electrolyte solution, to reduce the electrolyte solution's surface tension, and to reduce the intensity of acid mist breakout. (See, for example, U.S. Pat. Nos. 4,484,990 and 5,468,353). But a uniform thickness of the foam blanket can be difficult to maintain. Acid mist may escape in areas where the foam is too thin. Also, the foam layer may become too thick and thus interfere with the electrical contacts of the cell.
Other attempts to reduce acid mist generation have included providing a fabric screen or some kind of a cover over the electrode plates where the plates extend above the surface of the electrolyte solution. (See, for example, WO/0065 131). These attempts, however, have been ineffective in preventing acid mist generation.